1. Field of the Invention
The present invention is directed to an apparatus for processing a substrate wafer and, more particularly, to an apparatus for processing a substrate wafer wherein a temperature sensor having two sensor elements is provided for measuring the difference between the temperatures at the site of a susceptor and at a site between the susceptor and the substrate wafer itself.
2. Description of the Prior Art
In many semiconductor fabrication processes, it is important for the temperature of a substrate wafer being processed to be measured and/or regulated with an accuracy of better than one percent. This technical problem also arises outside of semiconductor fabrication in the processing of other substrate wafers; for example, during heat-treatment and/or the application of thin layers to glass or plastic substrate wafers.
In semiconductor fabrication, the temperatures in question are often greatly in excess of 1000.degree. C. The problem arises both in so-called RTP reactors in which the temperature is varied rapidly at 50.degree. C. per second and in reactors where the temperature is varied only slowly wherein the substrate wafer is in thermal equilibrium with its surroundings.
Various methods have been proposed for measuring the temperature of a substrate wafer being processed (see, for example, Solid State Technology, December 1996, pages 34 to 36). In optical methods, the intensity of radiation from or through the substrate wafer is measured. The accuracy of these methods is limited by the fact that the radiation intensity emitted or transmitted depends on the optical properties of the substrate wafer. These properties differ greatly from one substrate wafer to another, and expensive calibration methods are therefore necessary. Furthermore, the optical properties can change during the processing operation; for example, during the deposition of a layer.
It has also been proposed to provide a susceptor in the reactor. A susceptor is a body which is arranged next to the substrate wafer. The mass of the susceptor depends on the thermal requirements of the application. In RTP reactors, where it is necessary for the temperature to be varied rapidly, the susceptor has a low mass and therefore generally a small thickness. In epitaxy reactors or layer deposition reactors, where it is necessary to have a uniform temperature, susceptors generally have a high mass and therefore large thicknesses. The terms hot plate, hot liner and heat shield, or just shield, are common alternatives to the term susceptor in this field. In order to measure the temperature of a substrate wafer, the temperature of the susceptor is measured. It is assumed that the temperature of the susceptor is substantially equal to the temperature of the substrate wafer. However, although the measurement of the temperature of the susceptor, with optical methods or using a thermocouple, is very accurate, the assumption that the temperature of the substrate wafer is equal to the temperature of the susceptor is, at the very least, not very reliable.
Lastly, acoustic methods have also been proposed. So far however, these have not been found to be satisfactory in terms of either their accuracy or their reproducibility.